1. Field of the Invention
The present invention is related to a display apparatus such as a plasma display panel (PDP) and digital micromirror device (DMD), and more specifically, to a display apparatus achieving gradation display by using a plurality of subfield images.
2. Related Art
A display apparatus of a PDP and a DMD makes use of a subfield method, which has binary memory, and which displays a dynamic image possessing half tones by temporally superimposing a plurality of binary images that have each been weighted. The following explanation deals with PDP, but applies equally to DMD as well.
A PDP subfield method is explained using FIGS. 1, 2 and 3.
Now, consider a PDP with pixels lined up 10 across and 4 vertically, as shown in FIG. 3. Let the respective R, GB of each pixel be 8 bits, assume that the brightness thereof is rendered, and that a brightness rendering of 256 gradations (256 gray scales) is possible. The following explanation, unless otherwise stated, deals with a G signal, but the explanation applies equally to R, B as well.
The portion indicated by A in FIG. 3 has a signal level of brightness of 128. If this is displayed in binary, a (1000 0000) signal level is added to each pixel in the portion indicated by A. Similarly, the portion indicated by B has a brightness of 127, and a (0111 1111) signal level is added to each pixel. The portion indicated by C has a brightness of 126, and a (0111 1110) signal level is added to each pixel. The portion indicated by D has a brightness of 125, and a (0111 1101) signal level is added to each pixel. The portion indicated by E has a brightness of 0, and a (0000 0000) signal level is added to each pixel. Lining up an 8-bit signal for each pixel perpendicularly in the location of each pixel, and horizontally slicing it bit-by-bit produces a subfield. That is, in an image display method, which utilizes the so-called subfield method, by which one field is divided into a plurality of differently weighted binary images, and displayed by temporally superimposing these binary images, a subfield is one of the divided binary images.
Since each pixel is displayed using 8 bits, as shown in FIG. 2, 8 subfields can be achieved. Collect the least significant bit of the 8-bit signal of each pixel, line them up in a 10xc3x974 matrix, and let that be subfield SF1 (FIG. 2). Collect the second bit from the least significant bit, line them up similarly into a matrix, and let this be subfield SF2. Doing this creates subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7, SF8. Needless to say, subfield SF8 is formed by collecting and lining up the most significant bits.
FIG. 4 shows the standard form of a 1 field PDP driving signal. As shown in FIG. 4, there are 8 subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7, SF8 in the standard form of a PDP driving signal, and subfields SF1 through SF8 are processed in order, and all processing is performed within 1 field time. The processing of each subfield is explained using FIG. 4. The processing of each subfield constitutes setup period P1, write period P2 and sustain period P3. At setup period P1, a single pulse is applied to a sustaining electrode, and a single pulse is also applied to each scanning electrode (There are only up to 4 scanning electrodes indicated in FIG. 4 because there are only 4 scanning lines shown in the example in FIG. 3, but in reality, there are a plurality of scanning electrodes, 480, for example.). In accordance with this, preliminary discharge is performed.
At write period P2, a horizontal-direction scanning electrodes scans sequentially, and a predetermined write is performed only to a pixel that received a pulse from a data electrode. For example, when processing subfield SF1, a write is performed for a pixel represented by xe2x80x9c1xe2x80x9d in subfield SF1 depicted in FIG. 2, and a write is not performed for a pixel represented by xe2x80x9c0.xe2x80x9d
At sustain period P3, a sustaining pulse (driving pulse) is outputted in accordance with the weighted value of each subfield. For a written pixel represented by xe2x80x9c1,xe2x80x9d a plasma discharge is performed for each sustaining pulse, and the brightness of a predetermined pixel is achieved with one plasma discharge. In subfield SF1, since weighting is xe2x80x9c1,xe2x80x9d a brightness level of xe2x80x9c1xe2x80x9d is achieved. In subfield SF2, since weighting is xe2x80x9c2,xe2x80x9d a brightness level of xe2x80x9c2xe2x80x9d is achieved. That is, write period P2 is the time when a pixel which is to emit light is selected, and sustain period P3 is the time when light is emitted a number of times that accords with the weighting quantity.
As shown in FIG. 4, subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7, SF8 are weighted at 1, 2, 4, 8, 16, 32, 64, 128, respectively. Therefore, the brightness level of each pixel can be adjusted using 256 gradations, from 0 to 255.
In the B region of FIG. 3, light is emitted in subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7, but light is not emitted in subfield SF8. Therefore, a brightness level of xe2x80x9c127xe2x80x9d (=1+2+4+8+16+32+64) is achieved.
And in the A region of FIG. 3, light is not emitted in subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7, but light is emitted in subfield SF8. Therefore, a brightness level of xe2x80x9c128xe2x80x9d is achieved.
A display apparatus as described above which displays image with gradations by using a plurality of subfields has a problem that pseudo-contour noise appears while displaying a motion picture. Pseudo-contour noise is noise that occurs from the human visual characteristics. It appears due to the human visual characteristics and a characteristics of subfield display in a display apparatus which displays image with gradations by using the subfield method. That is, it is a phenomenon, whereby, when a person moves his eyes, a subfield that differs from an original gradation is projected on a retina, and therefore the original gradation is misperceived. Pseudo-contour noise is explained below.
Assume that regions A, B, C, D from the state shown in FIG. 3 have been moved one pixel width to the right as shown in FIG. 5. Thereupon, the viewpoint of the eye of a person looking at the screen also moves to the right so as to follow regions A, B, C, D. Thereupon, three vertical pixels in region B (the B1 portion of FIG. 3) will replace three vertical pixels in region A (A1 portion of FIG. 5) after one field. Then, at the point in time when the displayed image changes from FIG. 3 to FIG. 5, the eye of a human being is cognizant of region B1, which takes the form of a logical product (AND) of B1 region data (0111 1111) and A1 region data (1000 0000), that is (0000 0000). That is, the B1 region is not displayed at the original 127 level of brightness, but rather, is displayed at a brightness level of 0. Thereupon, an apparent dark borderline appears in region B1. If an apparent change from xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d is applied to an upper bit like this, an apparent dark borderline appears.
Conversely, when an image changes from FIG. 5 to FIG. 3, at the point in time when it changes to FIG. 3, a viewer is cognizant of region A1, which takes the form of a logical add (OR) of A1 region data (1000 0000) and B1 region data (0111 1111), that is (1111 1111). That is, the most significant bit is forcibly changed from xe2x80x9c0xe2x80x9d to xe2x80x9c1,xe2x80x9d and in accordance with this, the A1 region is not displayed at the original 128 level of brightness, but rather, is displayed at a roughly 2-fold brightness level of 255. Thereupon, an apparent bright borderline appears in region A1. If an apparent change from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d is applied to an upper bit like this, an apparent bright borderline appears.
In the case of a dynamic image only, a borderline such as this that appears on a screen is called pseudo-contour noise (xe2x80x9cpseudo-contour noise seen in a pulse width modulated motion picture displayxe2x80x9d: Television Society Technical Report, Vol. 19, No. 2, IDY95-21, pp. 61-66), causing degradation of image quality.
As technology for reducing this pseudo-contour noise, there is a display apparatus disclosed in Japanese Patent Laid-Open Publication No. 09-258689 or 10-39830. The display apparatus of No. 09-258689 attempts to reduce pseudo-contour noise by selecting a different modulating signal every n pixels, and performing different modulation every n pixels using the selected modulation signal. However this apparatus performs pseudo-contour noise reduction processing for an entire image, therefore it has a problem that displayed image quality over the entire image is degraded since reduction processing is performed for an area in which a pseudo-contour noise does not originally appear.
Further, the display apparatus of No. 10-39830 detects a dynamic area (motion picture area) of an image, and reduces pseudo-contour noise by performing modulation processing on every pixel in this area. However, this apparatus performs pseudo-contour noise reduction processing for an entire dynamic area, and therefore it performs pseudo-contour noise reduction processing even for an area where pseudo-contour noise dose not appear. Consequently quality of displayed image is degraded when an entire image is viewed.
The present invention has as an object to provide a detector, which solves for the above-described problems, for detecting pseudo-contour noise that spuriously appears in a dynamic area of an image in a display apparatus which displays gradations by using a plurality of subfield images.
The present invention also has an object to (provide a display apparatus suitable for a plasma display panel etc, for reducing appearance of pseudo-contour noise by making use of the pseudo-contour noise detector.
In a first aspect of the invention, a detector is provided for detecting appearance of pseudo-contour noise. The pseudo-contour noise appears spuriously when displaying a motion picture in a manner that gradation display is performed by using a plurality of subfields into which one field of input image is divided. The detector comprises a noise calculating unit to compare a value of one pixel with values of pixels peripheral to the one pixel in each subfield for each pixel of an input image, and to calculate a noise quantity based on said comparison result. The noise quantity indicates the probability of pseudo-contour noise appearance in the input image displayed.
The noise calculating unit may comprise a pixel comparing unit and a noise determining unit. The pixel comparing unit may compare a value of one pixel with values of pixels peripheral to the one pixel in each subfield for each pixel of an input image, and detect the difference of pixel value among those pixels in each subfield for each pixel from the result of the comparison. The noise determining unit may determine the noise quantity based on the difference of pixel value from the pixel comparing unit.
Further, the detector may comprise an exclusion area detecting unit and a excluding unit. The exclusion area detecting unit may detect an area in which the pseudo-contour noise is not expected to occur in the input image. The excluding unit may exclude the area detected by the exclusion area detection unit from area in which the noise quantity is calculated by the noise calculating unit.
The advantage of the detector according to the invention is that it is possible to specify both the magnitude of the probability of pseudo-contour noise appearance and the area in an image in which pseudo-contour noise is likely to be generated.
In a second aspect of the invention, A display apparatus is provided for displaying an input image with gradations by using a plurality of subfields into which one field of said input image is divided. The display apparatus comprises the detector to detect appearance of pseudo-contour noise and a pseudo-contour noise reducing unit. The pseudo-contour noise reducing unit reduces the pseudo-contour noise for an area in which there is a probability of the pseudo-contour noise appearing based on the results by the detector.
The pseudo-contour noise reducing unit may control gradation of the displayed image to reduce appearance of said pseudo-contour noise.
Further, the pseudo-contour noise reducing unit may reduce the pseudo-contour noise by performing a predetermined modulation to an image area in which the appearance of pseudo-contour noise is expected by the detector.
The advantage of the display apparatus according to the invention is that the appearance of pseudo-contour noise can be reduced and the degradation of picture quality can be prevented when displaying image with the subfield method.